The present invention relates to a light guide for display devices of the head-mounted or head-up type.
The purpose of display devices of the head-mounted or head-up type is to present an image, generated, for example, by a microdisplay, superimposed on a background in such a way that an observer will be able to see the image generated by the microdisplay without having to distract his attention from the background itself.
There are already present on the market a number of solutions for presenting to an observer the image generated by a microdisplay.
Micro Optical Inc. offers on the market two solutions, described in documents U.S. Pat. No. 5,715,377, U.S. Pat. No. 5,886,822, U.S. Pat. No. 6,023,372 and U.S. Pat. No. 6,091,546. A first solution is an optical system that can be clipped onto an ordinary pair of spectacles, the said system being made up of a display that generates the image, an optical system that forms a virtual image of the microdisplay at an appropriate distance, and a light guide that presents the image to the eye of the user. This solution is not rigorously definable as xe2x80x9csee-throughxe2x80x9d, in so far as the image that is returned to the observer is not superimposed onto the background. Since, however, the background is visible in the entire field of view around the image presented to the observer, said solution is defined as xe2x80x9csee-aroundxe2x80x9d. The display presents, however, a limited field of view and can thus return only low-resolution images (11xc2x0 in the horizontal direction). A second solution is of the xe2x80x9cenglobed opticsxe2x80x9d type, in which a display generates the image which, through a system of lenses and prisms englobed directly in the lens of the spectacles of the user, is presented to the latter at an appropriate distance. A half-reflecting deviating prism set at 45xc2x0 with respect to the surface of the lens and in the centre of the field of view causes the image to be presented to the user superimposed on the background. The external overall dimensions of this solution are quite small; however, use of the deviating prism at 45xc2x0 implies a thickness of the lenses of the spectacles which is at least equal to the output pupil of the system. Since the said output pupil increases with the field of view of the display in order to guarantee a sufficient tolerance of positioning of the eye (referred to hereinafter as xe2x80x9ceye-motion boxxe2x80x9d or EMB), the solution proposed is usable only for limited fields of view (11xc2x0 in the horizontal direction) and low resolution (320xc3x97240 pixels).
The document US-A1-20010033401 describes a solution that envisages the use of a light guide, in which the beams emitted by the display (geometrically coupled within the light guide without interposition of any optical devices), after traversing an appropriate stretch in total internal reflection between the walls of the light guide are extracted by means of a holographic optical device. This system is light and presents small overall dimensions and is therefore particularly suitable for a solution of the xe2x80x9cclip-onxe2x80x9d type, i.e., one that envisages clipping the device on the user""s spectacles. However, the optics of the system of a holographic type is markedly non-axial and, hence, is affected by numerous aberrations, in particular by a trapezoidal distortion of the field. Although the process of production of the hologram has been conceived so as to reduce to the minimum the problems linked to non-axiality and, hence, to aberration, such problems would, however, be of an even greater degree in the case where the intention were to use the proposed solution for a field of view of a greater amplitude ( greater than 160xc2x0). Furthermore, as is known, holographic optical systems are markedly sensitive to variations both in the angle of incidence and in the wavelength. Dependency upon the angle of incidence entails different levels of efficiency of extraction for the light that propagates in the guide at different angles, this necessarily limiting the field of view that can be used in the proposed solution. Dependency upon the wavelength entails different levels of extraction efficiency for light of different wavelengths, this limiting use of this solution to displays of the monochromatic type. Use of this solution in displays with wider fields of view and/or in colour displays entails a significant reduction in the efficiency of extraction of the hologram, i.e., the fraction of light that propagates in the guide which is deviated towards the eye of the observer.
The document U.S. Pat. No. 6,169,613 describes a solution with holographic optics comprising a light guide and three diffractive optical elements. The first optical element couples within a light guide the image generated by an appropriate optical system, causing the light beams to propagate within the guide by total internal reflection. The second optical element rotates through 90xc2x0 the direction of propagation of the beams that impinge thereon, at the same time enabling expansion of the pupil in the direction of propagation previous to the rotation performed by the holographic element. The third element extracts the beams that impinge thereon, reducing their angle of propagation to a value lower than that of the angle of total internal reflection, and at the same time enables expansion of the pupil in the direction of propagation subsequent to the rotation performed by the second holographic optical element. This solution enables amplification of the size of the output pupil in two directions substantially orthogonal to one another and lying in a plane perpendicular to the direction of sight of the observer. The efficiency of rotation of the second diffractive optical element increases in the direction of propagation in the guide so as to guarantee that the portion of beam rotated at each subsequent reflection will be constant. Likewise, the efficiency of extraction of the third diffractive optical element increases in the direction of propagation in the guide so as to guarantee that the portion of beam extracted from the guide at each successive reflection will be constant. The solution with three holographic optical elements becomes necessary when the specifications of the field of view and/or EMB of the display are such as to require an expansion of the pupil in two directions. Alternatively, it would be possible to increase the dimensions of the output pupil of the coupling optical device and use a two-hologram solution. However, the increase in the output pupil of the coupling optical device typically involves the use of a more complex, costly and cumbersome optical chain, as is known to persons skilled in the branch of optical design.
An alternative solution is described in the patent application FR8906721, which describes a combination optical device consisting of a transparent light guide with a plane end forming an access optically coupled to the collimation optical system and two large plane-parallel faces, and, within the guide, a number n greater than 1 of half-reflecting mirrors set parallel to one another and inclined with respect to the straight section of the guide. The light coupled inside the guide propagates by total internal reflection until it encounters the first of said half-reflecting mirrors. Part of the light is deflected by the mirror and extracted by the light guide, whilst another part continues its propagation within the guide until it encounters the second half-reflecting mirror. The reflectance decreases from the first to the last mirror so as to guarantee a uniformity of the light extracted from the guide.
The patent application FR8906721 basically repeats the patent U.S. Pat No. 4,099,841, which describes a head-up display. The combiner of the display described in this invention is based upon a light guide that includes three or more partially reflecting layers arranged parallel to one another and used basically for the same purposes as those described in the patent application FR8906721.
The solution presented in the document FR8906721 is taken up by the patent U.S. Pat. No. 5,153,774, with the addition of new inventive content.
A solution, which is substantially identical to the one described in the patent application FR8906721 both in terms of the description and in terms of the claims, is proposed in the recent patent application WO01/95027-A1.
The solution proposed in the patent application FR8906721 represents a valid alternative to the holographic solution presented in the document U.S. Pat. No. 6,169,613. The absence of holographic elements represents an advantage for application thereof in colour displays, especially in the case of a wide field of view.
However, the solution with parallel-half reflecting mirrors presents a certain number of disadvantages as compared to the holographic solution. The holographic optical elements described in U.S. Pat. No. 6,169,613 may be gratings of the surface-relief type. Gratings of this type can be replicated conveniently using moulding technologies, for example, injection moulding, casting or hot embossing. This means that the holographic solution is very well suited to large-scale production such as the ones typically adopted for consumer electronics. Instead, the solution with half-reflecting mirrors typically involves fabrication procedures that are decidedly more complex. Some possible manufacturing solutions are described in the patent application WO01/95027-A1. One of these solutions envisages bonding of a number of plates of glass, each coated with a half-reflecting layer having a different value of reflectance and subsequently cut in a direction that is substantially oblique to the planes of the glass plates. The process is all the more complex and costly, the greater the number of half-reflecting mirrors. For a certain specified value of EMB, the number of the mirrors is the higher, the smaller the thickness of the guide. This means that the solution is not competitive when the aim is to make a guide having a thickness that is, for instance, smaller than 3 mm. On the other hand, thick light guides ( greater than 5 mm) may entail the need to vary the reflectance of the coating not only between adjacent mirrors but also on the individual mirror in order to prevent the alignment of a number of mirrors in certain sectors of the field of view from leading to a lack of uniformity of luminance within the field of view itself.
The purpose of the present invention is to provide a light guide for display devices of the head-mounted or head-up type which will enable the drawbacks of the known solutions to be overcome.
According to the present invention, the above purpose is achieved by a light guide having the characteristics forming the subject of the main claim. In particular, the solution proposed enables both the limits of performance deriving from the use of diffractive or holographic optical devices and the limits on technologies of fabrication and on the performance deriving from the use of half-transparent mirrors englobed within the light guide to be overcome. This is achieved through the use of a half-reflecting grating set in the proximity of one of the faces of the light guide, which can be built using a moulding process and coated with a half-reflecting layer deposited in a prevalent way on the optically active surfaces of the grating.
The solution according to the present invention further enables light guides having thicknsses smaller than 5 mm to be made, also resorting to inexpensive technological solutions.